The present invention relates to phase change memory, and more particularly, relates to methods and apparatus for controlling the programming of the phase change memory.
Phase change memory uses a thermal process in which a special material, such as a chalcoginide glass, is switched between amorphous and crystalline states. The crystalline and amorphous states of chalcogenide glass have dramatically different electrical resistivity. The amorphous, high resistance state may represent a binary 0, while the crystalline, low resistance state may represent a 1. Phase change memory may be programmed by applying a current which causes the phase change memory to heat and change state. If the phase change memory is heated to the melting temperature of the phase change material, it will become amorphous while if it is heated to a lower, crystallizing temperature, it will become crystalline. The melting temperature and crystallization temperature are material dependent.
Phase change memory (PCM) is an emerging storage class memory technology. Three dimensional (3D) PCM stacks have been demonstrated in order to increase the storage capacity and improve access latency per footprint area but data retention due to the resistance loss in the amorphous phase of the material is considered to be an important challenge in phase change memory technologies. This wear out mechanism must be managed to extend the lifetime of the PCM. This challenge becomes more prominent in stacked environments where multiple cells are in close proximity which increase localized heat and propagation to adjacent areas.